The present invention relates to a method of galvanic structuring of endless pressing bands.
More particularly it relates to a method in accordance with which a steel band with certain alloying ingredients, treated in an acid bath and provided with a welding seam is used for galvanic structuring of endless pressing bands.
Pressing bands are used for continuous manufacture of laminated products and for coating of wood chip boards. The surface structure of the pressing bands is transferred in a pressing step under the action of pressure and temperature to the laminated product or to the coated wood chip board. Fine grain structures, wood pore representation and other geometrical designs are used as impression structures. The structure supporting surface is in all cases the outer surface of the pressing band which is additionally coated with a wear resistant hard chromium layer to maintain the load during the pressure application in so-called multi-layer presses. Frequently in the pressing bands also the inner side is provided with such a hard chromium layer. During the operation also an upper band and a lower band are utilized. They run synchronously with one another, and the coated chip board or the respective press laminate is treated between the associated runs of both pressing bands.
Such endless pressing bands are usually produced from steel band material with a thickness of 1-2 mm. Both ends of the steel band are connected with one another by means of conventional welding processes, for example laser or plasma welding. After the formation of the welding seam the welding seam zone must be treated. The treatment is performed by grinding the welding beads with simultaneous removal of fusing residues in the heating zone or the material bead region. After the preparation works in the welding seam zone the endless row bands are finely ground over the whole outer and inner surfaces, and thereby each band obtains its own material thickness with high tolerance accuracy. In some cases, further surface treatments are provided, for example mirror finishing.
Conventionally, for the steel for the steel band material mainly austenitic and martensitic steels are used with predetermined alloying ingredients. In the case of the austenitic steel material the alloying ingredients include the following average values: carbon 0.1%, silicium 0.6%, manganese 1.4%, chromium 17.5%, nickel 7.5%. In the case of austenitic steels the material values include a pulling strength of approximately 1,200 N/mm.sup.2, a yield point of 980 N/mm.sup.2, an elasticity limit of 600 N/mm.sup.2 and an elongation of 22%.
In the martensitic steels the alloying ingredients include the following average values: carbon 0.05%, silicium 1%, manganese 1%, chromium 13%, nickel 4%, and titanium 0.3%. The material values include a pulling strength of 1,080 N/mm.sup.2 a yield point of 1,000 N/mm.sup.2, an elasticity limit of 850 N/mm.sup.2 and an elongation of 5%.
Sample pressing bands of martensitic and austenitic alloys with one or several welding seams within an endless band were provided for experiments first with an etching-resistant printing ink in form of a design pattern having selectively a grain design or wood pore design. The sample band was suspended in a conventional acid immersion bath, and therefore as known a metal removing takes place on the metal-blank points which are free from the printing ink. Thereby the later printing image is produced. This metal removing process positively exposes the welding seam zones. After finishing the metal removing process in the immersion bath, a non-uniform structural image is recognized on all pressing bands in the welding seam zone. Undesired bead-like elevations in the welding seam zones are visible, each possibly can be traced back to the fact that in the direct welding seam zone a lower etching speed in the acid bath takes place than in the remaining welding seam-free regions.
In other sample bands instead of the bead-shaped elevations in the welding seam regions, wedge-shaped depressions in the welding seam zones were formed. This can probably be traced back to the fact that at these locations the etching speed was higher than in the remaining regions. The explanation for these measuring results in the same immersion bath but for different band samples can be probably found in the fact that different hardness conditions existed over the cross-section of the welding seam. Despite numerous comparative tests of endless steel band samples obtained commercially, advancements in the practice to produce proper structure image in the region of the welding seam zones were obtained. Invariably these bands lead during later pressing of the end products to visible marks on the press laminates or the coated chip plates.
In accordance with another known structuring methods disclosed in the German reference DE-OS 2,950,795 the attempt is made to avoid the above described problems in the region of the welding seam zones. In this method a metal layer is applied by galvanization on the welding seam zone and is thicker than the later etching engraving. The disadvantage of this method is however that the application of the galvanic layer which practically serves as a later structure supporting layer is expensive. Since the ductility of the galvanic layer on the pressing band is not always error free in many cases, there is the danger that at least in partial regions this can lead to relief formation of the galvanic layer on the pressing band. This is promoted by the fact that the pressing bands during their latter utilization in the multi-layer heating presses are subjected to strong pulling and bending loads.
In accordance with another method disclosed in the German reference DE-OS 3,337,962, after the formation of the surface structure of the pressing band by means of an electrolytic bath, an additional layer in the welding seam region is unnecessary. However, there certain problems with respect to the adhesion of the galvanically applied structure figures on the steel sheet exist.